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Negative differential conductivity effects in semiconductors Torrens, Alain Bernard

Abstract

The thesis is concerned principally with Gunn-effect oscillations, low-frequency oscillations due to field-enhanced trapping (FET), and the interaction of the Gunn effect with traps. In the first part of the thesis, the theory of these phenomena is developed in a form which is unified in that it applies to all three independently of their physical cause i.e. field-enhanced interband transfer or FET. The theory of the quasi-steady-state is treated, in which interlevel electron transfer is assumed to be "fast" compared with electric field changes. The theory of the Gunn effect and of FET instabilities is then reviewed in this framework, as well as experimental observations. Some complements to previous theoretical knowledge are included, such as a discussion of stationary electric distributions, a rigorous derivation of the smalls-signal equivalent circuit of a Gunn diode and its application to a domain-stability criterion. Various computational simulations were performed, of which the following seem not to have been reported previously. Gunn-domains oscillation in a diode with doping increasing linearly from cathode to anode was simulated. It was known experimentally that such a device has an interesting voltage-tunability, and this was confirmed and explored. The simulation of a subcritical diode showed that its negative differential conductance is associated with the launching of accumulation layers in synchronism with the voltage modulation, which was not previously made clear. High-field domains due to field-enhanced trapping were found to grow exponentially with time; some relations between their dynamic behaviour and the trapping parameters are inferred from the results. Field-independent trapping was found to have little effect on steadily-propagating Gunn domains; in a domain, traps are depleted. The result show, however, that cumulative trapping near the cathode, and at the anode, might alter the oscillation. Field-enhanced trapping can result in the trapping of the Gunn domain at the anode, These two results are important in connection with the performance of practical Gunn oscillators in that the material should not contain too many traps; impurities causing such traps should not enter the material by contamination from the electrodes at the surface. For the experimental part of the thesis, diodes were cleaved, from slices of N-type, high-resistivity gallium arsenide onto which ohmic contacts had been applied. Above a threshold field of about 0.3 MV/m, some of these diodes exhibited small current oscillations due to high-field domains propagating at some 1 m/s. The diode-current transients revealed the existence of a trap level just below the Fermi level. Illumination strongly enhanced the field-enhanced trapping. The use of the electro-optic effect to probe the electric-field distribution in the diode was investigated. The-experiments showed that, in a non-ohmic diode biased in the high-resistance direction, there exists a high-field region near the cathode.

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